This invention relates to infrared photodiode imaging array detectors and, more particularly, to a mercury - cadmium - telluride photodiode array detector having a composite structure of two overlapping metallization layers wherein the layers have openings of different sizes to enable selective areas of the HgCdTe substrate surface to undergo inversion.
An infrared photo diode imaging array converts incident infrared radiation to an electrical current. The magnitude of the current generated by each of the diodes in the array bears a direct relationship to the flux density of the infrared radiation impinging on the infrared detecting diode.
The conversion efficiency of the individual diode elements in an imaging array is a function of the area of the p-n junction which comprises the diode. As the surface area of the n-type material in contact with the p-type material increases so does the conversion efficiency. However, a problem arises in that increasing the area of the n-type region also increases the amount of thermal leakage current, or dark current, generated by the diode, with a consequent reduction in the diode signal-to-noise ratio. This results in a degradation of the overall image seen by the array of diodes.
The n-type regions within the diode array are formed typically by ion implantation techniques, wherein a suitable n-type material is implanted by a stream of particles directed into the crystalline lattice of a p-type substrate. This implantation process by its nature, however, introduces a problem in that the material implanted within the substrate may cause damage to the crystalline lattice thereof. One result of this damage may be a further increase in the thermally induced diode leakage current.